Book No /ices mid Revieivs. 
157 
of olijections and facts, the conclusion is reached that few, if any, 
organisms exist, wliicli do not i)ossess some feature, which this 
theory utterly fails to explain. They certainly clearly indicate 
their own leanings, viz., that all species throw off very distinctive 
variations at greater or less periods, and that upon these variations 
natural selection acts. 
Hybridisation and the fertility of hybrids. — Here of course 
at present only provisional conclusions can be drawn, yet this 
chapter is one of the most inteiesling of the whole book. 
When considering the colouration of organisms Messrs. 
Dewar and Finn are very definite, almost aggressive f tJi the 
writer' s opiJiion, riohtly so) as to the superiority . of the theories 
or conclusions drawn from Field and Forest observations, over 
the emanations from study and museum. It certainly must be 
obvious that the dead body can reveal little beyond osteological 
and general anatomical data, and that other deductions drawn 
therefrom are limited indeed as factors of biological research. 
Again I must express the regret that our space does not 
permit of the review we fain would give, but we must quote the 
pertinent (for aviculturists) paragraphs with which they conclude 
their book : 
" Tke real inaker.s of .species are the inherent properties of protoplasm 
"and the laws of variation and heredity. These determine the uatme of 
" the organism ; natural selection and the like factors merely decide for 
"each particular organism whether it shall survive and give rise to a 
" species. 
" The way in which Natural .Selection does its work is comparatively 
" easy to understand. But this is only the fringe of the territory which 
"we call evolution. We seem to be tolerably near a solution of the 
"problem of the causes of the survival of any particular imitation. This, 
" however is merely a side issue. The real problem is the causes of 
"variations and mutations, or, in other words, how species originate. At 
"present our knowledge of the causes of variation and mutation is prac- 
"tically 7iil. We do not even know along what particular lines mutations 
"follow. We have yet to discover whether one mutation invariably leads 
"to another along the same lines — in other words, whether nnitating 
" organisms behave as though they had behind them a force acting in a 
"definite direction. The solution of these problems seems afar off. The 
"hope of solving them lies, not in the speculations in which biologists of 
"to-day are so fond of indulging, but in ol)servation and experiment, 
" especially the last. 
